1. Field of the Invention
The present invention relates to a battery test circuit and, more particularly, to a battery test circuit that gauges battery condition on the basis of a relative change in potential when a load is applied to the battery.
2. Description of the Related Art
A battery level tester is useful in various battery-powered electronic applications for testing charge on a battery and informing a user of the battery voltage level. For example, the battery level tester may be used in a battery level indicator to warn when failure of the battery is impending. The battery level tester may also be used in a battery charger controller to determine whether a battery is charged, thereby preventing overcharging and damaging of the battery.
One technique for battery level testing is accomplished by the apparatus shown in FIG. 1, which includes a comparator 150, a voltage divider 110 connected to receive a battery voltage Vcc and a reference generator 140. The voltage divider 110 includes two resistors 112 and 114, connected in series. A node 124 between the resistors furnishes a tap line for accessing a divided voltage. The battery voltage Vcc is applied to the voltage divider 110 and the node 124 connects a divided voltage line 120 to one input terminal, the inverting input, of the comparator 150. The other comparator terminal, the noninverting input, receives the reference voltage from the reference generator 140 on line 122 so that, when the battery is low and the divided voltage [V.sub.d ] on line 120 is less than the reference voltage [V.sub.ref ] on line 122, the comparator output 134 goes high, indicating an impending battery failure. One problem with this technique is that a stable reference generator is difficult, and relatively expensive, to implement in silicon. Another problem with this technique is that components, for example resistors (112 and 114), in the voltage divider have fixed impedance values so that it is only practical for testing batteries of a single nominal voltage. It is desirable to apply the battery level indicator regardless of the voltage of the battery. For example, it is useful to apply a battery level indicator for usage in testing both 5 volt and 3 volt batteries, as well as batteries of other voltages. One solution to this problem is to replace the reference generator on the chip with an external pin to supply the reference voltage. However, the external pin and additional external resistors disadvantageously increase the size and cost of the battery level indicator.
An additional implementation of a battery level indicator employs an analog to digital converter and a reference generator for testing the battery level. This approach is disadvantageously expensive for many applications.
What is desired is an inexpensive battery level indicator for implementation in silicon that is useful for testing batteries of various nominal voltages.